The present invention is related to an electromechanical transducer using piezoelectric polymeric substances. It is also related to a process for manufacturing such transducer.
The piezoelectric effect generated by this type of transducer is due to flexion when the transducer structure considered is a bimorph structure.
The polymeric substances used in transducers allow flexional deformations or deformations of the elongation-compression type of the transducer. Such transducers are polarized by passing through a rolling mill in the presence of an electric field. The instant invention is particularly related to transducer bodies obtained by conjointly rolling (co-laminating) two layers of polymeric substance, said layers being welded, or bonded, to each other by means of a polymeric substance which is rendered conductive by a convenient ingredient.
According to a known method, at least two superimposed polymer layers are rolled, which layers do not adhere to each other at the end of this operation. According to this technique, the intermediary electrode required for effecting the polarization is a fluid electrode. In this case a conductive bonding agent may be added after the rolling operation, so as to produce a usable intermediary electrode and a monolithic structure. When such operation is performed after rolling in the presence of an electric field, certain piezoelectric properties of the polymers will be impaired on account of the pressure which must be applied at elevated temperature with a view to obtaining efficient bonding.
When it is desired to privilege, or promote, the piezoelectric coefficients perpendicularly to the flexional axis, it is preferred to use so-called "mono-drawn" piezoelectric blades. According to one known method, the manufacturing of transducers of this type consists in co-laminating (i.e., co-jointly rolling) at least two polymer blades while polarizing the same, the rolling cylinders (or rollers) acting as electrodes for applying the electric polarizing field.
When it is desired to obtain mutually opposed remanent polarizations it is advantageous to apply the voltage required for creating the electric field between the intermediary electrode and the ground potential, in which case the rolling cylinders are connected to the ground.
When carrying out the rolling operation the most exacting problem resides in effecting this operation in such a manner that the intermediary electrode retains its electric properties in spite of the stresses to which it is submitted by the action of the rolling cylinders. Indeed the forces exerted by said cylinders range from 40 to 100 tons; the drawing or stretching ratio may reach values as high as 3 or 4, and consequently rupturing may occur in the mass of the electrode. Another problem is raised by the adherence obtained by bonding (gluing) the polymer blades to the intermediary electrode, since the expansion coefficients of the materials constituting the transducer are substantially different from each other.
With a view to overcoming these drawbacks the present invention aims, inter alia, at providing an intermediary electrode made of a polymeric substance rendered conductive by incorporating therein a convenient material which is easily integrated in the mass of said substance. This polymeric substance acts as a central (or intermediary) electrode as well as an adhesive between two blades of the transducer.